I think all positions on the line are important, but when you have a pocket passer with a quick release like Mac I don't worry as much about edge rushers getting to him since he can step up and he's not going to sit back there forever, but I do worry about pressure up the middle preventing him from stepping up (and getting in his face affecting a pass). This is what we watched with Brady for years; unless someone had a monster game like Von Miller where the blocking barely bothered them, he was getting the pass out of his hands before an edge rusher could get there. Not to mention the run game of course. So I don't think the Pats can just run Onwenu and ??? out there.
I'm with others; I'm sure if BB thought he could get this guy in the mid-second and there was a trade for such a pick available, he would have done it, but if there's a pretty flat draft outside of the top 15 or so then you never know if he'll be there at that point. And with an interior OL their track record is pretty good.
BB has found good OL later but he's also found good defensive players late in the draft too.